1. Field of the Invention
The present invention relates to a method for forming a portion of a workpiece, such as a cylinder or shell, and an apparatus therefor, especially the method and apparatus for forming the portion of the workpiece by spinning to form a changed diameter portion of the workpiece, such as a reduced diameter portion of the cylinder.
2. Description of the Related Arts
As for the method for forming a changed diameter portion of a cylindrical member (hereinafter, simply referred to as a cylinder), Japanese Utility-model Laid-open Publication No. 61-110823 discloses a method for forming a cone portion and a body portion by increasing or reducing a diameter of the cylinder to produce a case for holding a catalyst, and reducing a diameter of an open end portion of the case except for the body portion thereof, by a spinning process, to form the other cone portion and a conduit connected thereto in a body. In Japanese Patent Laid-open Publication No. 3-226327, there is disclosed a method for pressing a tubular member longitudinally by a press die to be formed into an approximately conical shape, then rotating the tubular member and pressing a spinning roll onto the outer surface of the portion formed into the conical shape to perform the spinning process, thereby to form an opening portion of a pressure case or the like.
In the mean time, with respect to an outer shell of a catalytic converter or a muffler of an automotive vehicle, it is demanded to produce it easily, and mount it easily in the vehicle, and it is desired to produce it integrally from a metal tube. In this situation, it has been desired to form the reduced diameter portion to be formed on the end portion of the tubular portion, into an unusual shape, such as the one having an offset axis or an oblique axis inclined to a central axis of the cylinder. According to prior methods for forming the cylinder or shell by the spinning process, however, the reduced diameter portion was formed to be coaxial with the main body of the cylinder. In order to produce the cylinder, the main body of which is not coaxial with the reduced portion, therefore, the cone portion (reduced diameter portion) as shown in the right side in FIG. 1 of the above Publication No. 61-110823 was formed by the press working, and then the cone portion was connected to the case body by welding or the like. According to those methods, however, the produced cylinder can not be expected to be so strong, comparing with that of the integral construction. Furthermore, they need the connecting process, different from the forming process, so that it is difficult to produce the cylinder by those methods. As a result, the manufacturing cost of the cylinder shall be increased, comparing with the cylinder of the coaxial type formed by the spinning process.